@CrazyMaster
Depending on what you mean by using the "Branch" twice, so far I believe the only instance in the series of a "Branch" being used repeatedly by someone is Touya after getting killed repeatedly to the point the ability/talent he stole called the
[The Undead Soldier], that gave him immortality, was exhausted and pushed him to the point of the "Branch" undoing his reincarnation and temporarily leaving his body as a form of pseudo death in the place of Touya.
In every other case so far throughout the series I believe, "Branches" only come out of Returners upon death. This was a set precedent, with Kou's dialogue quite some time ago where people who hoped to be reborn as some amazing returner ended up being reborn as mass murderers or worse and couldn't undo the fact afterwards unless they died. Aside from this, no side heavy side effects have been shown so far from Touya using a "Branch" repeatedly to turn back into a returner aside from being both mentally and physically exhausted from the process leading up to him having to do so.
It hasn't been mentioned or discussed at all so far by anyone, about what happens when a returner tries to use another "Branch" when they are already a returner. Its implied that nothing happens, as they are already returners and don't have another life past that to return to.